Pwm signal generator circuit and related integrated circuit

ABSTRACT

A PWM signal generator circuit includes a multiphase clock generator that generates a number n of phase-shifted clock phases having the same clock period and being phase shifted by a time corresponding to a fraction 1/n of the clock period. The PWM signal generator circuit determines for each switch-on duration first and second integer numbers, and for each switch-off duration third and fourth integer numbers. The first integer number is indicative of the integer number of clock periods of the switch-on duration and the second integer number is indicative of the integer number of the additional fractions 1/n of the clock period of the switch-on duration. The third integer number is indicative of the integer number of clock periods of the switch-off duration, and the fourth integer number is indicative of the integer number of the additional fractions 1/n of the clock period of the switch-off duration.

BACKGROUND Technical Field

The embodiments of the present description refer to solutions forgenerating a pulse-width modulation (PWM) signal.

Description of the Related Art

Generally, as shown in FIG. 1, a PWM signal is a periodic signal havinga given switching period T_(SW), wherein the PWM signal is set to highfor a given switch-on duration T_(ON) and low for a given switch-offduration T_(OFF), with:

T _(SW) =T _(ON) +T _(OFF)  (1)

Moreover, often is defined the duty cycle D of the PWM signal, withD=T_(ON)/T_(SW).

Such a PWM signal may be generated in various modes. For example, asshown in FIG. 1, one of the simplest solutions is based on an oscillatorcircuit generating a clock signal CLK and a counter configured toincrease a count value in response to the clock signal CLK. Thus, byusing a comparator circuit the PWM signal may be generated as a functionof the count value provided by the counter, e.g., by comparing the countvalue with given threshold values, e.g., indicative of the switch-onduration T_(ON) and the switching period T_(SW).

However, in such a (digital) implementation, the accuracy and resolutionof the PWM signal is limited by the clock period T_(CLK) (samplingfrequency) of the clock signal CLK. Moreover, by increasing the clockfrequency f_(CK)=1/T_(CLK) also the switching losses will increase.

In many applications, high resolution PWM signals are required orstrongly preferred. For example, this PWM signals may be used in manyapplications to control the average value of a voltage or current, suchas for wireless battery chargers, switching mode power converters, motorcontrol and lighting. For example, in such applications a half-bridge orfull bridge may be used to drive a resonant tank, usually comprising oneor more inductors and capacitors, wherein the electronic switches of thehalf-bridge or full bridge are driven by means of PWM signals.

In order to miniaturize the equipment, small inductors may be usedleading to a high working frequency. Thus, often a high-frequencymodulated waveform PWM signal with high precise resolution should beprovided in order to keep power consumption at acceptable values. Forexample, in a switching power supply, the output voltage is oftendirectly proportional to the PWM duty cycle. The smaller is theadjustment to the duty cycle, the smaller is the resulting change to theoutput, i.e., a more precise control of the output voltage that permitsto achieve a better accuracy level and system stability. Moreover,minimizing output voltage ripple means reduce noise levels.

An alternative solution for generating a PWM signal, in particular aHigh Resolution (HR) PWM signal, is based on the use of multiple clockphases, i.e., phase-shifted clock signals having the same frequency.

For example, FIG. 2 shows a possible circuit for generating multipleclock phases ϕ₀ . . . ϕ_(n), via a Delay Locked Loop (DLL).

Specifically, in the example considered the clock signal CLK generatedby an oscillator OSC is fed to a cascade of a plurality of (identical)delay stages DU₁ . . . DU_(n). Specifically, in the example considered,the first phase ϕ₀ corresponds to the clock signal CLK, and the otherphases ϕ₁ . . . ϕ_(n), correspond to the output signals of the delaystages DU₁ . . . DU_(n).

In the example considered, each of the delay stages DU₁ . . . DU_(n) hasa delay T_(DU) being programmable/settable as a function of a (voltageor current) control signal CTRL. For example, such delay stages DUhaving a variable delay may be implemented with an even number ofinverters, wherein one or more of the inverters charges a respectivecapacitance, such as a parasitic capacitance, connected to the output ofthe inverter. In this case, the control signal CTRL may be indicative ofthe current provided by the inverter to charge the respectivecapacitance, thereby varying the time until the following inverterswitches.

In the example considered, the last phase ϕ_(n) (having a given delayT_(D)=n·T_(DU) with respect to the clock signal CLK) and the clocksignal CLK is provided to a phase detector PD. The output of the phasedetector PD is fed to a regulator CP having at least an I (Integral)component, such as a charge pump, wherein the regulator CP provides atoutput the control signal CTRL. Optionally the control signal CTRL maybe passed through a loop filter LF.

Thus, essentially, the negative feedback loop, implemented by the blocksPD/CP/LF, synchronizes in time the last phase ϕ_(n) with the clocksignal CLK. If the delay cells DU are identical, all the clock phases ϕ₁. . . ϕ_(n) will have the same frequency f_(CLK), but are phase shiftedwith respect to the preceding phase by a delay of T_(DU)=T_(CLK)/n.

Such multiple clock phases may also be provided by a Phase Locked Loop(PLL) comprising a Voltage Controlled Oscillator (VCO) comprising aring-oscillator with a plurality of delay stages, wherein the PLL islocked to the frequency of a clock signal CLK. Also in this case, alocking of the PLL may be obtained by varying the delay introduced bythe delay stages, e.g., by varying via a bias circuit the currentprovided by the inverter stages implementing such delay stages, untilthe oscillator signal at the output of the VCO corresponds to the clocksignal CLK. Thus, each delay stage of the VCO may provide a respectiveclock phase, which is phase shifted by a given fraction of the period ofthe clock signal CLK.

For example, FIG. 3 shows exemplary waveforms for the phases ϕ₁ . . .ϕ₁₆ in case n=17, wherein the last phase ϕ₁₇=ϕ₀=CLK is not shown in theFigure.

Accordingly, as shown in FIG. 4, while a counter and respectivecomparator circuit may provide a coarse PWM signal (having a plurality kof clock cycles of the clock signal CLK), the additional clock phases ϕ₁. . . ϕ_(n) may be used to add a fine tuning to the coarse PWM signal,which essentially permits to add fractions T_(DU) of the clock signalCLK to the coarse PWM signal. For example, such a solution is describedin document U.S. Pat. No. 7,206,343 B2, the content thereof beingincorporated herein by reference for this purpose.

For example, the fraction may be added to the coarse PWM signal by:

-   -   directly combining, e.g., by using one or more logic (e.g., OR)        gates, the coarse PWM signal with a given selected clock phase        ϕ, or    -   as described in document U.S. Pat. No. 7,206,343 B2, indirectly        by passing the coarse PWM signal through additional delay stages        and combining the coarse PWM signal with the delayed PWM signal,        e.g., via a logic (e.g., OR) gate, wherein the additional delay        stages introduce the same delay Thu as the delay stages DU₁ . .        . DU_(n), e.g., by biasing the additional delay stages with the        same control signal CTRL as the delay stages DU₁ . . . DU_(n).

Thus, assuming that the counter (and a respective comparator circuit)provides a coarse PWM signal having a switching period T_(SW)=i·T_(CLK)and a switch-on duration of T_(ON)=k·T_(CLK), with 0≤k≤i, the final PWMsignal may have a switching period T_(SW)=i·T_(CLK) and a switch-onduration T_(ON)=k·T_(CLK)+l·T_(CLK)/n, with 0≤1<n. Thus, the switch-onduration T_(ON) of the PWM signal may be selected by setting the integervalues of the parameters k and l. Thus, essentially the use of anadditional DLL or PLL permits to vary the switch-on duration T_(ON), orin general the duty cycle D, with a higher precision, while theswitching period T_(SW) remains constant.

BRIEF SUMMARY

Considering the foregoing, various embodiments of the present disclosureprovide solutions for generating a PWM signal.

According to one or more embodiments, a PWM signal generator circuit isprovided having the distinctive elements set forth in the followingdescription. The embodiments also concern a corresponding integratedcircuit.

Various embodiments of the present disclosure relate to a PWM signalgenerator circuit configured to generate a Pulse-Width Modulated signalhaving a given switching duration comprising a switch-on duration and aswitch-off duration.

In various embodiments, the PWM signal generator circuit comprises amultiphase clock generator configured to generate a given number n ofphase-shifted clock phases having the same clock period and being phaseshifted by a time corresponding to a fraction 1/n of the clock period.

In various embodiments, the PWM signal generator circuit is configuredto:

determine for each switch-on duration a first and a second integernumber, the first integer number being indicative of the integer numberof clock periods of the switch-on duration and the second integer numberbeing indicative of the integer number of the fractions 1/n of the clockperiod of the switch-on duration in addition to the integer number ofclock periods of the switch-on duration, and determine for eachswitch-off duration a third and a fourth integer number, the thirdinteger number being indicative of the integer number of clock periodsof the switch-off duration or the integer number of clock periods of theswitching duration, and the fourth integer number being indicative ofthe integer number of the fractions 1/n of the clock period of theswitch-off duration in addition to the integer number of clock periodsof the switch-off duration.

For example, in various embodiments, the PWM signal generator circuitmay receive at input the first, second third, and fourth integer number.

In various embodiments, the PWM signal generator circuit comprises aclock switching circuit, a timer circuit, a phase accumulator circuitand a toggle circuit.

In various embodiments, the clock switching circuit is configured togenerate a timer clock signal by selecting one of the phase-shiftedclock phases as the timer clock signal as a function of a selectionsignal.

For example, in various embodiments, the clock switching circuitcomprises:

-   -   for each of the phase shifted clock phases a respective        transmission gate, and wherein each transmission gate is        configured to generate a respective gated clock phase as a        function of the selection signal; and    -   a combinational logic circuit configured to generate the timer        clock signal by combining the gated clock phases.

In various embodiments, the timer circuit comprises one or more countersand one or more comparators, wherein the timer circuit is configured to:

-   -   during a switch-on duration, vary a first count value in        response to the timer clock signal and generate a first trigger        when the first count value reaches the first integer number, and    -   during a switch-off duration, vary a second count value in        response to the timer clock signal and generate a second trigger        when the second count value reaches the second integer number.

For example, the timer circuit may comprise a single counter configuredto generate the first count value and the second count value. In thiscase, the third integer number may be indicative of the integer numberof clock periods of the switch-off duration, and the single counter maybe reset at the beginning of each switch-on duration and each switch-offduration. Alternatively, the third integer number may be indicative ofthe integer number of clock periods of the switching duration, and thesingle counter may be reset only at the beginning of each switch-onduration.

In various embodiments, the phase accumulator circuit is configured togenerate the selection signal by:

-   -   during a switch-on duration, increasing the selection signal by        the second integer number, and    -   during a switch-off duration, increasing the selection signal by        the fourth integer number.

Generally, the variation of the selection signal may occur at anyinstant during the respective switch-on or switch-off period. However,preferably, the phase accumulator circuit is configured to generate theselection signal by:

-   -   in response to the first trigger, increasing the selection        signal by the second integer number, and    -   in response to the second trigger, increasing the selection        signal by the fourth integer number.

In various embodiments, the toggle circuit is configured to:

-   -   in response to the first trigger, setting the PWM signal to low,        and    -   in response to the second trigger, setting the PWM signal to        high.

In such embodiments, the timer circuit operates thus with an adaptiveclock signal resulting from a switching/combination of the phase-shiftedclock phases.

The inventors have observed that the switching of the clock phases maythus occur while the previous clock phase is high, resulting in a lossof an edge used to increase the timer circuit.

Accordingly, in order to compensate this missing edge, in variousembodiments, the PWM signal generator circuit is configured to:

-   -   during a switch-on duration, determine whether the second        integer number is smaller than n/2, and in case the second        integer number is smaller than n/2, increase the first count        value for a single clock cycle of the timer clock signal by two;        and    -   during a switch-off duration, determine whether the fourth        integer number is smaller than n/2, and in case the fourth        integer number is smaller than n/2, increase the second count        value for a single clock cycle of the timer clock signal by two.

Alternatively, the PWM signal generator circuit may be configured to:

-   -   during a switch-on duration, determine whether the second        integer number is smaller than n/2, and in case the second        integer number is smaller than n/2, decrease the first integer        number by one; and    -   during a switch-off duration, determine whether the fourth        integer number is smaller than n/2, and in case the fourth        integer number is smaller than n/2, decrease the third integer        number by one.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The embodiments of the present disclosure will now be described withreference to the drawings, which are provided purely to way ofnon-limiting example and in which:

FIG. 1 shows an example of a PWM signal;

FIG. 2 shows an example of a circuit generating multi-phase clocksignals;

FIG. 3 shows an example of the waveforms of clock phases provided by thecircuit of FIG. 2;

FIG. 4 shows an example of the fine tuning of the switch-on duration ofa PWM signal by means of multi-phase clock signals;

FIG. 5 shows an embodiment of the fine tuning of both the switch-onduration and the switch-off duration of a PWM signal by means ofmulti-phase clock signals;

FIGS. 6A and 6B show embodiments of a timer circuit in accordance withthe present disclosure;

FIG. 7 shows exemplary waveforms generated by the timer circuits ofFIGS. 6A and 6B;

FIG. 8 shows an embodiment of a PWM generator circuit; and

FIGS. 9A, 9B, 10A, 10B, 10C, 11A, 11B, 12A, 12B, 12C and 12D showvarious details of the circuits of FIGS. 6A, 6B and 8.

DETAILED DESCRIPTION

In the ensuing description, various specific details are illustrated toenable an in-depth understanding of the embodiments. The embodiments maybe provided without one or more of the specific details, or with othermethods, components, materials, etc. In other cases, known structures,materials, or operations are not shown or described in detail so thatvarious aspects of the embodiments will not be obscured.

Reference to “an embodiment” or “one embodiment” in the framework ofthis description is meant to indicate that a particular configuration,structure, or characteristic described in relation to the embodiment iscomprised in at least one embodiment. Hence, phrases such as “in anembodiment”, “in one embodiment”, or the like that may be present invarious points of this description do not necessarily refer to one andthe same embodiment. Moreover, particular conformations, structures, orcharacteristics may be combined in any adequate way in one or moreembodiments.

The references used herein are only provided for convenience and hencedo not define the sphere of protection or the scope of the embodiments.

In FIGS. 5 to 12 described below, parts, elements or components thathave already been described with reference to FIGS. 1 to 4 aredesignated by the same references used previously in these figures. Thedescription of these elements has already been made and will not berepeated in what follows in order not to burden the present detaileddescription.

As explained in the foregoing, various embodiments of the presentdescription relate to a PWM signal generator circuit configured togenerate a high resolution PWM signal. In particular, in variousembodiments, the PWM signal generator circuit is configured to receive aplurality of clock phases ϕ₀ . . . ϕ_(n) and generate both the risingand the falling edges of the PWM signal as a function of these clockphases ϕ₀ . . . ϕ_(n), thereby controlling both the PWM duty cycle andthe PWM frequency with a higher resolution.

FIG. 5 shows the general operation of a first embodiment.

In the embodiment considered, the PWM signal generator circuit receivesthe first clock phases ϕ₀ (and/or the last clock phase ϕ_(n)=ϕ₀) and theintermediate clock phases ϕ₁ . . . ϕ_(n-1). In some embodiments, the PWMsignal generator circuit includes a multiphase clock generator thatgenerates the various clock phases, which may include any multiphaseclock generator configured to generate the clock phases describedherein. Possible solutions for generating such clock phases are alreadydescribed in the introduction of the present disclosure, and therelevant description applies in its entirety (see in particular thedescription of FIG. 2). That is, in some embodiments, the multiphaseclock generator circuit of various embodiments of the present disclosuremay be as described, for example, with respect to FIG. 2.

Moreover, in the embodiment considered, the PWM signal generator circuitis configured to generate a PWM signal, wherein:

-   -   the switching duration T_(SW) may be set to        T_(SW)=i·T_(CLK)+j·T_(CLK)/n; and    -   the switch-on time T_(ON) may be set to        T_(ON)=k·T_(CLK)+l·T_(CLK)/n.

In various embodiments, the parameters i, j, k and l integer values,wherein the parameters i, j, k and l may be programmable.

Specifically, in the example shown in FIG. 5, it is assumed that n=17,e.g., the PWM signal generator circuit receives the clock phases ϕ₀ . .. ϕ₁₆, and the PWM signal generator circuit is configured to generate aPWM signal with:

T _(SW) =i·T _(CLK)+10·T _(CLK)/17=T _(i)+10·T _(CLK)/17,

-   -   a duty cycle of 50% (i.e., T_(ON)=T_(OFF)=T_(SW)/2), i.e.,

T _(ON) =T _(OFF) =T _(i)/2+5·T _(CLK)/17.

In the example considered, it will be assumed for simplicity that i isan even number, and k=p=i/2.

Specifically, in the embodiment considered, the PWM signal generatorcircuit is configured to use during the first switch-on period T₁ thephase ϕ₀ as clock signal for the digital counter counting the timeperiod T_(i)/2=k·T_(CLK), and (as will be described in greater detail inthe following) the PWM signal generator circuit adds at the end afraction of 5/17 of the period T_(CLK) by using the phase ϕ₅.

However, instead of then tracking the accumulation of the variousfractions, the PWM signal generator circuit uses then during thefollowing switch-off period T₂ the phase ϕ₅ (i.e., the phase used to addthe fraction) as clock signal for the timer circuit (i.e., the digitalcounter counting the time period p·T_(CLK)). Moreover, the PWM signalgenerator circuit adds at the end again the respective fraction of 5/17of the period T_(CLK) by using in this case the phase ϕ₁₀, insofar asthe phase ϕ₁₀ is shifted by a delay of 5·T_(CLK)/17 with respect to thephase ϕ₅.

Next, the PWM signal generator circuit use during the second switch-onperiod T₃ the phase ϕ₁₀ as clock signal for the digital counter countingthe time period k·T_(CLK), and the PWM signal generator circuit adds atthe end a fraction of 5/17 of the period T_(CLK) by using this time thephase ϕ₁₅, insofar as the phase ϕ₁₅ is shifted by a delay of5·T_(CLK)/17 with respect to the phase ϕ₁₀.

Similarly, the PWM signal generator circuit use during the followingswitch-off period T₄ the phase ϕ₁₅ as clock signal for the digitalcounter counting the time period T_(CLK), and the PWM signal generatorcircuit adds at the end a fraction of 5/17 of the period T_(CLK) byusing this time the phase ϕ₃, insofar as the phase ϕ₃ is shifted by adelay of 5·T_(CLK)/17 with respect to the phase ϕ₁₅.

This operation continues also for the following switch-on and switch offperiods.

In various embodiments, the PWM generator circuit is thus configured togenerate a PWM signal, wherein:

-   -   the switch-on duration corresponds to        T_(ON)=k·T_(CLK)+l·T_(CLK)/12; and    -   the switch-off duration corresponds to        T_(OFF)=p·T_(CLK)+q·T_(CLK)/n.

In various embodiments, the parameter n (number of delay stages/phase)is fixed at a hardware level. However, the number n could also beprogrammable, e.g., by using in FIG. 2 a given fixed number of delaystages (e.g., 32) and selecting the n-th phase (and not necessarily thelast one) as feedback signal provided to the phase detector PD. In fact,in this way, the control loop will still be locked to the n-th phaseϕ_(n), with T_(DU)=T_(CLK)/n.

Thus, in various embodiments, the timer circuit of the PWM signalgenerator circuit (comprising the counter circuit and the comparatorcircuit) is configured to:

-   -   during a switch-on period T_(ON), increase a count value from a        reset value until the count value reaches the integer value k;        and    -   during a switch-off period T_(OFF), increase a count value from        a reset value until the count value reaches the integer value p.

However, in general, the timer circuit may also monitor the switchingduration T_(SW), i.e., the timer circuit of the PWM signal generatorcircuit (comprising the counter circuit and the comparator circuit) maybe configured to:

-   -   during a switch-on period, increase a count value from a reset        value until the count value reaches the integer value k; and    -   during a switch-off period, increase the count value used during        the switch-on period until the count value reaches the integer        value i.

Thus, in various embodiments, the PWM signal generator circuit isconfigured to determine the parameters k/l, and at least one of p/q, andi/j wherein:

-   -   in case of a switch-on period T_(ON), k corresponds to the        integer number of clock cycles of the clock signal CLK and l        corresponds to the integer number of the fractions 1/n of a        clock cycle of the clock signal CLK;    -   in case of a switch-off period T_(OFF), p corresponds to the        integer number of clock cycles of the clock signal CLK and q        corresponds to the integer number of the fractions 1/n of a        clock cycle of the clock signal CLK; and    -   in case of a switching period T_(SW), i corresponds to the        integer number of clock cycles of the clock signal CLK and j        corresponds to the integer number of the fractions 1/n of a        clock cycle of the clock signal CLK.

Specifically, in view of the above definitions:

T _(ON) =k·T _(CLK) +l·T _(CLK) /n  (2)

T _(OFF) =p·T _(CLK) +q·T _(CLK) /n  (3)

T _(SW) =T _(ON) +T _(OFF) =i·T _(CLK) +j·T _(CLK) /n  (4)

the integer values i and j are related to the integer values k, l, p andq according to the following equations:

-   -   in case (l+q)<n (without overflow):

i=k+p;j=l+q;  (5)

-   -   in case (l+q)>n (with overflow):

i=k+p+l;j=l+q−n.  (6)

Thus, in various embodiments, the PWM generator circuit is configured toreceive at least two of the parameters i, k and p, and at least two ofthe parameters j, l and q. For example, the PWM signal generator circuitmay directly receive the parameters k/l and/or p/q and/or i/j, such as:

-   -   data identifying (e.g., corresponding to) the parameters k/l;        and    -   data identifying (e.g., corresponding to) the parameters p/q.

Alternatively, the PWM signal generator circuit may receive other datapermitting a calculation of these parameters according to equations (5)and (6), such as:

-   -   data identifying the switching duration T_(SW), such as the        above-mentioned parameters i and j, and one of:        -   data identifying (e.g., corresponding to) the parameters            k/l;        -   data identifying (e.g., corresponding to) the parameters            p/q; or        -   data identifying the duty cycle

As shown in FIG. 6A, in various embodiments, the PWM signal generatorcircuit comprises a timer circuit 102 comprising a digital countercircuit 104 configured to vary (i.e., increase or decrease) an integercount value CNT in response to a clock signal CLK_TMR and a comparatorcircuit 106 configured to compare the count value CNT with a respectiveinteger comparison threshold.

As shown in FIG. 6A, the same counter 104 and comparator 106 may be usedfor both the switch-on period and the switch-off period by selecting,e.g., via a multiplexer 108, the parameter k or p as comparisonthreshold. Accordingly, by resetting the counter 104 via the signal atthe output of the comparator 106, the same counter 104 may be used tomonitor the switch-on period and the switch-off period. However, thecounter 104 may also be used to monitor the switch-on period and theduration T_(SW). For example, in this case, the multiplexer 108 mayreceive the parameters k and i, and the counter 104 may only be resetwhen the count value CNT reaches the value i.

Alternatively, as shown in FIG. 6B, a respective counter 104 a and 104 band comparator 106 a and 106 b may be used for the switch-on period andthe switch-off period, wherein the comparator 106 a compares a countvalue CNTa provided by the counter 104 a with the parameter k and thecomparator 106 b compares a count value CNTb provided by the counter 104ab with the parameter p.

In various embodiments, the timer circuit 102 is configured to generateone or more trigger signal when the output of the comparator indicatesthat the count value has reached the comparison threshold, e.g., byusing a signal EOC_TMR at the output of the comparator 106, orrespective signal EOC_TMRa and EOC_TMRb at the outputs of thecomparators 106 a and 106 b.

In the embodiments considered, the signal EOC_TMR (FIG. 6A) or thesignals EOC_TMRa and EOC_TMRb (FIG. 6B) are provided to a controlcircuit 110 with selects the clock signal CLK_TMR for timer circuit 102,in particular the counter 104 (104 a/104 b), as a function of:

-   -   during a switch-on period, the parameter 1; and    -   during a switch-off period, the parameter q.

Specifically, even when monitoring the end of the switching durationT_(SW), it is preferably to obtain, e.g., calculate according toequations (5) and (6), the parameter q, because this parameter indicatesthe additional fractions which have to be added with respect to theprevious switch-on period.

For example, the control circuit 110 may select the clock signal CLK_TMRby driving via a selection signal SEL1 a multiplexer 100 receiving atinput the clock phases ϕ₀ . . . ϕ_(n-1). Similarly, the control signalmay drive via a selection signal SEL2 a multiplexer 112 in order toselect either the parameter 1 or the parameter q, i.e., the selectionsignal indicates whether the current period is a switch-on period or aswitch-off period, and may thus also be used to drive the multiplexer108.

Specifically, in various embodiments, in response to a trigger in thesignal EOC_TMR (FIG. 6A) or the signals EOC_TMRa and EOC_TMRb (FIG. 6B),the control circuit 110 is configured to change the logic value of theselection signal SEL1:

-   -   during a switch-on period, as a function of the parameter l; and    -   during a switch-off period, as a function of the parameter q.

Specifically, in various embodiments, the control circuit also performsa modulo operation in order to maintain the selection signal SEL1between 0 and n−1. Accordingly, in response to a trigger in the signalEOC_TMR (FIG. 6A) or the signals EOC_TMRa and EOC_TMRb (FIG. 6B), thecontrol circuit 110 varies the selection signal SEL1:

-   -   during a switch-on period, SEL1=(SEL1+l) mod n; and    -   during a switch-off period, SEL1=(SEL1+q) mod n.

Thus, essentially, the control circuit 110 implements a phaseaccumulator circuit, which adds to the currently selected phase either 1or q, wherein the parameters q may be calculated, e.g., as shown inequations (5) and (6) as a function of the parameters j and n.

Finally, in various embodiments, the respective period (either aswitch-on or switch-off period) is terminated and the following periodis started with the next clock pulse (i.e., with the next rising orfalling edge based on which type of edge is used by the timer circuit102) of the selected clock phase.

Thus essentially, during a switch-on period T_(ON) the trigger signalEOC_TMR (or EOC_TMRa) is generated after a time k·T_(CLK), and bychanging the clock signal CLK_TMR the switch-on period is terminated,thereby starting the following switch-off period, after an additionaltime l/n·T_(CLK). Similarly, during a switch-off period T_(OFF) thetrigger signal EOC_TMR (or EOC_TMRb) is generated after a time p·T_(CLK)(which may be obtained, e.g., by resetting the counter 104 and waitingfor p cycles or by waiting until the count value reaches i), and bychanging the clock signal CLK_TMR the switch-off period is terminated,thereby starting the following switch-on period, after an additionaltime q/n·T_(CLK).

For example, this is shown in FIG. 7, wherein during a switch-on period,the timer circuit uses a clock phase CLK_TMR=ϕ_(x), and the triggersignal EOC_TMR is set after, e.g., k=9 periods of the phase ϕ_(x), e.g.,with the 10^(th) rising edge. In response to the trigger signal EOC_TMR(EOC_TMRa) the control circuit selects a new phase CLK_TMR=ϕ_(y) (withy=(x+l) mod n). Moreover, in response to the immediately following(e.g., rising) edge in the signal ϕ_(y), the PWM signal generatorcircuit terminates the switch-on period and starts the followingswitch-off period, thereby introducing an additional time correspondinga fraction l/n of the clock period.

In the embodiment considered, during the following switch-off period,the timer circuit uses then the clock phase CLK_TMR=ϕ_(y), and thetrigger signal EOC_TMR is set after, e.g., p=8 periods of the phaseϕ_(y), e.g., with the 9^(th) rising edge. In response to the triggersignal EOC_TMR (EOC_TMRb) the control circuit selects a new phaseCLK_TMR=ϕ_(z) (with z=(y+q) mod n). In response to the immediatelyfollowing (e.g., rising) edge in the signal ϕ_(z), the PWM signalgenerator circuit terminates the switch-off period and starts thefollowing switch-on period, thereby introducing an additional timecorresponding a fraction q/n of the clock period.

In the previous embodiments, the control circuit 110 is configured todrive the selection circuit 100 in order to changes the phase ϕ assignedto the clock signal CLK_TMR from the current phase ϕ(t) (e.g., ϕ₀) tothe next phase ϕ(t+1) (e.g., ϕ₅) in response to the signal EOC_TMR,thereby adding the fractions (l or q) at the end of the respectiveswitch-on or switch-off period.

However, in various embodiments, the switching from the current phase(kW to the next phase ϕ(t+1) may occur at any instant during therespective period. In this case, the control unit 110 may also beconfigured to either increase/decrease sequentially, e.g., in responseto the clock signal CLK_TMR, the selection signal SEL1 from the oldphase ϕ(t) to the new phase ϕ(t+1) (e.g., ϕ₀, ϕ₁, ϕ₂, ϕ₃, ϕ₄, ϕ₅) or byswitching directly to the new phase.

Generally, while reference has been made to periods of the clock signalCLK, indeed the phases ϕ₀ . . . ϕ_(n-1) may also have a different clockperiod T_(PLL), e.g., the frequency f_(PLL)=1/T_(PLL) may be a multipleof the clock frequency f_(CLK), e.g., by using a frequency divider inthe feedback loop of the phase ϕ_(n-1). Accordingly, in general:

-   -   the switch-on duration corresponds to        T_(ON)=k·T_(PLL)+l·T_(PLL)/n; and    -   the switch-off duration corresponds to        T_(OFF)=p·T_(PLL)+q·T_(PLL)/n.

FIG. 8 shows a second embodiment of a PWM signal generator circuit.

Specifically, in the embodiment considered, the PWM signal generatorcircuit comprises again a timer circuit 102, a clock switching circuit100′ and a control circuit/phase accumulator 110′.

Specifically, with respect to FIGS. 6A and 6B, the clock switchingcircuit 100′ is not implemented with a mere multiplexer, but with acircuit which directly generates, in response to the trigger signalEOC_TMR provided by the timer circuit 102, the clock signal CLK_TMR forthe timer circuit as a function of the selection signal SEL1 provided bythe control circuit 110′. Generally, as described in the foregoing, alsoany other trigger signal may be used to assign to the clock signalCLK_TMR a new clock phase as a function of the selection signal SEL1.

For example, a possible embodiment of the clock switching circuit 100′is shown in FIGS. 9A and 9B.

In the embodiment considered, the selection signal SEL1 (indicative ofthe next clock phase), is provided to a series of optional latches 1000configured to store the value of the signal SEL1 in response to thetrigger signal EOC_TMR. Substantially, these latches 1000 ensure thatthe circuit samples the value of the signal SEL1 only when a trigger inthe signal EOC_TMR is generated.

In the embodiment considered, each clock phase ϕ₀ . . . ϕ_(n-1) isprovided to a respective transmission gate (gated clock cells) 1002 ₀ .. . 1002 _(n) being enabled as a function of the selections signal SEL1or optionally the latched selections signal SEL1, thereby generatingrespective (gated) signals ϕ_(0_gtd) . . . ϕ_(n-1_gtd). For example, invarious embodiments, the selection signal comprises (n) bits SEL₀ . . .SEL_(n-1) and uses a one-hot encoding, wherein a given bit is associatedunivocally with a given clock phase ϕ₀ . . . ϕ_(n-1), i.e., only one ofthe bits SEL₀ . . . SEL_(n-1) is set and indicates that the respectiveclock phase ϕ₀ . . . ϕ_(n-1) may pass through the respectivetransmission gate 1002 ₀ . . . 1002 _(n-1), while the other clock phasesϕ₀ . . . ϕ_(n-1) cannot pass through the respective transmission gates1002 ₀ . . . 1002 _(n-1). In general, also other encoding schemes may beused for the selection signal (such as a binary encoding), and thetransmission gates may be driven via a decoder circuit configured togenerate the one-hot encoded drive signals for the transmission gates1002 ₀ . . . 1002 _(n-1) as a function of the selection signal SELL

As shown in FIG. 9B, the signals ϕ_(0_gtd) . . . ϕ_(n-1_gtd) are thenprovided to a combinational logic circuit 1004 configured to generate atoutput the clock signal CLK_TMR for the timer circuit 102 by combiningthe signals ϕ_(0_gtd) . . . ϕ_(n-1_gtd). For example, in variousembodiments the signals ϕ_(0_gtd) . . . ϕ_(n-1_gtd) are combined via alogic OR operation, e.g., implemented with a cascaded structure of aplurality of OR gates OR1, OR2, OR3, etc.

FIG. 10A shows the operation of the clock switching circuit 100′ at theexample of a selection signal SEL1 having in sequence the value k, x andy, thereby activating (in response to the trigger signal EOC_TMR) insequence the clock phases ϕ_(k_gtd), ϕ_(x_gtd) and ϕ_(y_gtd).

Thus, in case the selection signal SEL1 changes, the clock signalCLK_TMR switches from a first clock phase to a second clock phase inresponse to the selection signal.

Specifically, as shown in FIG. 10B, when the second clock phase((k_(x-gtd)) goes to high (rising edge), while the first clock phase(ϕ_(k_gtd)) is still high, the resulting clock signal CLK_TMR will havea single clock pulse with a duration being greater than the clock periodT_(PLL) of the clock phases ϕ₀ . . . ϕ_(n-1), thereby essentially losinga clock cycle.

Usually this occurs when the respective fraction l or q is smaller thann/2.

Conversely, as shown in FIG. 10C, when the second clock phase(ϕ_(y-gtd)) goes to high (rising edge), while the first clock phase(d)_(t)d) is low, the resulting clock signal CLK_TMR will have a singleclock pulse, with a duration being smaller than the clock period T_(PLL)of the clock phases ϕ₀ . . . ϕ_(n-1). Usually this occurs when therespective fraction l or q is greater than n/2.

Thus, the lost clock edge (FIG. 10B) should be taken into account inorder to correctly determine the duration of the respective timeinterval. Specifically, in various embodiments, in case a clock cycle islost, i.e., the respective fraction l or q is smaller than n/2, the PWMsignal generator circuit is configured to increase the timer circuit 102by an additional clock cycle, i.e., the timer 102 is increase by 2 andnot only 1 for a single clock cycle.

FIG. 11A shows a possible embodiment of the timer circuit 102.

Specifically, in the embodiment considered, the counter 104 isimplemented with an accumulator comprising:

-   -   a register 1040 providing at an output the count value CNT,        wherein the register 1040 is configured to store a signal REG_IN        at a respective input in response to the clock signal CLK_TMR;        and    -   a digital adder 1042, configured to generate the signal REG_IN        at the input of the register 1040 by adding an increment value        INC to the count value CNT.

In the embodiment considered, the increment value INC may be set eitherto “1” or “2”, e.g., via a multiplexer 1044. Specifically, the selectionis driven via a selection signal SEL3 provided by the control circuit110 (or similarly by the control circuit 110′).

Specifically, in the embodiment considered, the control circuit 110comprises:

-   -   a digital comparator 1100 configured to determine whether the        fraction value l or q of the current switch-on or switch-off        period is greater than n/2; and    -   a circuit 1102 configured to generate a selection signal SEL3 as        a function of the comparison signal generated by the comparator        1100 and a trigger signal indicating the start of a new        switch-on or switch-off period, such as the signal EOC_TMR or,        in the general case, as a function of the comparison signal        generated by the comparator 1100 and a generic trigger signal        whose length is one CLK_TMR cycle and generated in any        appropriate instant during the switch-on or switch-off period.

Specifically, in the embodiment considered, the multiplexer 112 alreadyprovide the fraction value for the current period, wherein the selectionsignal SEL2 indicates whether the current period is a switch-on orswitch-off period. Accordingly, the comparator 1100 may receive at inputthe signal provided by the multiplexer 112 and thus generates acomparison signal indicating whether the fraction value l or q isgreater than n/2. Specifically, the circuits 110 and 112 are configured:

-   -   when the signal at the output of the comparator indicates that        the fraction l or q (based on the current period) is greater        than n/2 or the trigger signal (e.g., EOC_TMR) is not set, drive        the multiplexer 1044 via the signal SEL3 in order to selected        the value “1”, whereby the accumulator 1040/1042 is increased in        response to the clock signal CLK_TMR by “1”; and    -   when the signal at the output of the comparator indicates that        the fraction l or q (based on the current period) is smaller        than n/2 and the trigger signal (e.g., EOC_TMR) is set, drive        the multiplexer 1044 via the signal SEL3 in order to selected        the value “2”, whereby the accumulator 1040/1042 is increased in        response to the clock signal CLK_TMR by “2”.

Accordingly, substantially, the timer circuit 104 is configured toincrease for one clock cycle of the signal CLK_TMR (i.e., a single cyclefor each switch-on or switch-off period) the count value by two (“2”)when the fraction l or q (based on the current period) is smaller thann/2.

Conversely, FIG. 11B shows that a similar result may be obtained byadapting directly the threshold value used by the comparator 106.

Specifically, in the embodiment considered, the increment value INC isalways set to “1”, and an additional digital subtractor is providedwhich is configured, e.g., via a multiplexer 1048, to:

-   -   subtract the value “1” from the current threshold selected by        the multiplexer 108 (k or p); or    -   maintain the threshold value, e.g., by subtracting the value “0”        from the current threshold selected by the multiplexer 108 (k or        p).

In general, the embodiments may also be combined, i.e., during aswitch-on duration may be implemented either the “plus-two” mechanism(FIG. 11A) or the adaption of the threshold k (FIG. 11B), and during aswitch-off duration may be implemented either the “plus-two” mechanismor the adaption of the threshold p.

Accordingly, in the embodiments considered, the circuits 1100/1102inform the timer circuit 102 that a counting edge has been missed orwill be missed due to clock combination shown in FIG. 9B. This missingedge information (i.e., the signal SEL3) can be computed by the controlcircuit/phase accumulator machine 110/110′ that controls the fine delayselection and generates the phase selection change SEL1 (indicative ofthe next clock phase to be used for fine tuning of PWM signal). In fact,if the new phase selection selects a clock having its rising edgeappearing during the on-time of the running clock, the combined CLK_TMRwill have a longer on-time and the edge of the next selected clockphase, used in the clock combination circuitry of FIG. 9B, will bemissed. This happens if the phase selection change is smaller than thehalf of number of available phases i.e., this occurs when the respectivefraction l or q is smaller than n/2 (e.g., └17/2┘=8).

Using this clock change property, the timer may be incremented by “1” or“2”, or the threshold of the comparator 106 may be adapted with respectto this internal flag generated as shown in FIG. 11A or 11B.

In various embodiments, the PWM signal is switched in response to thenext rising edge of the new clock phase, i.e., the selected clock phaseϕ_(0_gtd) . . . ϕ_(n-1_gtd) of the following switch-on or switch-offperiod. However, the PWM signal may also be changed in response to therising edge of the trigger signal EOC_TMR in the case of a SELL signalgenerated in any appropriate instant during the given time slot/period.

For example, as shown in FIG. 8, the PWM signal generator circuit maycomprise a toggle circuit 114 configured to generate the PWM signal as afunction of the signals ϕ_(0_gtd) . . . ϕ_(n-1_gtd) and the triggersignal EOC_TMR.

Generally, any suitable circuit may be used to toggle the level of thePWM signal in response to the signal EOC_TMR (or EOC_TMRa and EOC_TMRb)and the new clock phase.

For example, FIG. 12A shows an embodiment of the toggle circuit 114.Specifically, the toggle circuit 114 comprise a rising edged detectorcircuit. Specifically, in the embodiment considered, the toggle circuitcomprises for each of the signals ϕ_(0_gtd) . . . ϕ_(n-1_gtd) arespective rising edge detector 1140 ₀ . . . 1140 _(n-1), which isenabled as a function of the signal EOC_TMR.

Specifically, as shown in FIGS. 12B, 12C and 12D, in response to therising edge of the current clock phase (e.g., ϕ_(k_gtd) in FIG. 12C),the signal EOC_TMR will be set after a brief delay. In response to thetrigger in the signal EOC_TMR, the circuit 100′ will switch to the newclock phase (e.g., ϕ_(x_gtd) in FIG. 12C). Thus, no additional risingedge of the old clock signal (e.g., ϕ_(k_gtd) in FIG. 12C) occurs. Thus,in response to the following rising edge in the new clock phase (e.g.,ϕ_(x_gtd) in FIG. 12C) the respective edge detector 1140 will set itsoutput (e.g., to high), because also the signal EOC_TMR is still set.

Accordingly, in the embodiment considered, the output of the variousrising edge detector 1140 ₀ . . . 1140 _(n-1) may be connected to acombinational logic circuit, e.g., implementing a logic OR function(FIG. 12A shows schematically a logic OR gate OR4, that may correspondto the last OR gate of a chain of OR gates, e.g., comprising in cascade6 OR gates having three inputs, 2 OR gates having 2 inputs and the ORgate OR4) for this purpose but, generally speaking, it can beimplemented with a different number and topology of gates as a result ofa different balancing process with respect to speed and to the number ofclock phases), which generates at output a trigger signal TRIGindicating that the logic level of the PWM signal has to change.

Accordingly, in the embodiment considered, the signal TRIG may be usedto drive a flip-flop FF1 in order to invert the output of the flip-flopFF1, wherein the PWM signal is generated as a function (and preferablycorresponds to) the signal at the output of the flip-flop FF1.

For example, in the embodiment considered, the flip-flop FF1 isimplemented with a D-type flip-flop, receiving at the data terminal Dvia an inverter INV1 the inverted output signal of the flip-flop FF1,thereby inverting the output of the flip-flop FF1 in response to thetrigger signal TRIG.

Of course, without prejudice to the principle of the disclosure, thedetails of construction and the embodiments may vary widely with respectto what has been described and illustrated herein purely by way ofexample, without thereby departing from the scope of the presentdisclosure, as defined by the ensuing claims.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. A Pulse-Width Modulated (PWM) signal generator circuit, comprising: amultiphase clock generator configured to generate phase-shifted clockphases having a same clock period and being phase shifted by a timecorresponding to a fraction of the clock period, the PWM signalgenerator circuit configured to: generate a PWM signal having a givenswitching duration including a switch-on duration and a switch-offduration, determine for each switch-on duration a number of clockperiods of the switch-on duration and a number of the fractions of theclock period of the switch-on duration, and determine for eachswitch-off duration a number of clock periods of the switch-offduration, and a number of the fractions of the clock period of theswitch-off duration.
 2. The PWM signal generator circuit of claim 1,further comprising: a clock switching circuit configured to generate atimer clock signal by selecting one of the phase shifted clock phases asthe timer clock signal based on a selection signal.
 3. The PWM signalgenerator circuit according to claim 2, further comprising: a timercircuit including one or more counters and one or more comparators, thetimer circuit configured to: during the switch-on duration, vary a firstcount value in response to the timer clock signal and generate a firsttrigger when the first count value reaches a first integer number, thefirst integer number indicative of the number of clock periods of theswitch-on duration, and during the switch-off duration, vary a secondcount value in response to the timer clock signal and generate a secondtrigger when the second count value reaches a second integer number, thesecond integer number indicative of the number of clock periods of theswitch-off duration; a phase accumulator circuit configured to generatethe selection signal by: during the switch-on duration, increasing theselection signal by a third integer number, the third integer numberindicative of the number of fractions of the clock period of theswitch-on duration, and during the switch-off duration, increasing theselection signal by a fourth integer number, the fourth integer numberindicative of the number of fractions of the clock period of theswitch-off duration; and a toggle circuit configured to: in response tosaid first trigger, set the PWM signal to low, and in response to saidsecond trigger, set the PWM signal to high.
 4. The PWM signal generatorcircuit according to claim 3, configured to receive at an input thefirst, second, third, and fourth integer numbers.
 5. The PWM signalgenerator circuit according to claim 3, configured to: during theswitch-on duration, determine whether the third integer number issmaller than n/2, where n is equal to a number of the phase-shiftedclock phases, and in response to determining the third integer number issmaller than n/2, increase the first count value for a single clockcycle of said timer clock signal by two; and during the switch-offduration, determine whether the fourth integer number is smaller thann/2, and in response to determining the fourth integer number is smallerthan n/2, increase the second count value for a single clock cycle ofsaid timer clock signal by two.
 6. The PWM signal generator circuitaccording to claim 3, configured to: during the switch-on duration,determine whether the third integer number is smaller than n/2, where nis equal to a number of the phase-shifted clock phases, and in responseto determining the third integer number is smaller than n/2, decreasethe first integer number by one; and during the switch-off duration,determine whether the fourth integer number is smaller than n/2, and inresponse to determining the fourth integer number is smaller than n/2,decrease the second integer number by one.
 7. The PWM signal generatorcircuit according to claim 3, wherein the timer circuit includes asingle counter configured to generate the first count value and thesecond count value, and wherein the single counter is reset at thebeginning of each switch-on duration and each switch-off duration. 8.The PWM signal generator circuit according to claim 3, wherein the timercircuit includes a single counter configured to generate the first countvalue and the second count value, and wherein the single counter isreset only at the beginning of each switch-on duration.
 9. The PWMsignal generator circuit according to claim 3, wherein the phaseaccumulator circuit is configured to generate the selection signal by:in response to the first trigger, increasing the selection signal by thesecond integer number, and in response to the second trigger, increasingthe selection signal by the fourth integer number.
 10. The PWM signalgenerator circuit according to claim 3, wherein the clock switchingcircuit includes: for each of the phase shifted clock phases arespective transmission gate, each transmission gate configured togenerate a respective gated clock phase based on the selection signal;and a combinational logic circuit configured to generate the timer clocksignal by combining the gated clock phases.
 11. An integrated circuit,comprising: a PWM signal generator circuit configured to: receive anumber of phase-shifted clock phases having a same clock period andbeing phase shifted by a time corresponding to a fraction of the clockperiod; generate a PWM signal having a given switching durationincluding a switch-on duration and a switch-off duration; determine foreach switch-on duration a number of clock periods of the switch-onduration and a number of the fractions of the clock period of theswitch-on duration; and determine for each switch-off duration a numberof clock periods of the switch-off duration, and a number of thefractions of the clock period of the switch-off duration.
 12. Theintegrated circuit according to claim 11, wherein the PWM signalgenerator circuit includes: a clock switching circuit configured toselect one of the phase shifted clock phases as a timer clock signalbased on a selection signal.
 13. The integrated circuit according toclaim 12, wherein the PWM signal generator circuit includes: a timercircuit including one or more counters and one or more comparators, thetimer circuit configured to: during the switch-on duration, vary a firstcount value in response to the timer clock signal and generate a firsttrigger when the first count value reaches a first integer number, thefirst integer number indicative of the number of clock periods of theswitch-on duration, and during the switch-off duration, vary a secondcount value in response to the timer clock signal and generate a secondtrigger when the second count value reaches a second integer number, thesecond integer number indicative of the number of clock periods of theswitch-off duration; a phase accumulator circuit configured to generatethe selection signal by: during the switch-on duration, increasing theselection signal by a third integer number, the third integer numberindicative of the number of fractions of the clock period of theswitch-on duration, and during the switch-off duration, increasing theselection signal by a fourth integer number, the fourth integer numberindicative of the number of fractions of the clock period of theswitch-off duration; and a toggle circuit configured to: in response tothe first trigger, set the PWM signal to low, and in response to thesecond trigger, set the PWM signal to high.
 14. The integrated circuitaccording to claim 12, wherein the PWM signal generator circuit isconfigured to: during the switch-on duration, determine whether thethird integer number is smaller than n/2, and in response to determiningthe third integer number is smaller than n/2, increase the first countvalue for a single clock cycle of the timer clock signal by two; andduring the switch-off duration, determine whether the fourth integernumber is smaller than n/2, and in response to determining the fourthinteger number is smaller than n/2, increase the second count value fora single clock cycle of the timer clock signal by two.
 15. Theintegrated circuit according to claim 12, wherein the PWM signalgenerator circuit is configured to: during the switch-on duration,determine whether the third integer number is smaller than n/2, and inresponse to determining the third integer number is smaller than n/2,decrease the first integer number by one; and during the switch-offduration, determine whether the fourth integer number is smaller thann/2, and in response to determining the fourth integer number is smallerthan n/2, decrease the second integer number by one.
 16. The integratedcircuit according to claim 12, wherein the timer circuit includes asingle counter configured to generate the first count value and thesecond count value, and wherein the single counter is reset at thebeginning of each switch-on duration and each switch-off duration.
 17. Amethod, comprising: receiving a number of phase-shifted clock phaseshaving a same clock period and being phase shifted by a timecorresponding to a fraction of said clock period; generating a PWMsignal having a given switching duration including a switch-on durationand a switch-off duration; determining for each switch-on duration anumber of clock periods of the switch-on duration and a number of thefractions of the clock period of the switch-on duration; and determiningfor each switch-off duration a number of clock periods of the switch-offduration, and a number of the fractions of the clock period of theswitch-off duration.
 18. The method of claim 17, further comprising:selecting one of the phase shifted clock phases as a timer clock signalbased on a selection signal.
 19. The method of claim 18, furthercomprising: during the switch-on duration, varying a first count valuein response to the timer clock signal and generating a first triggerwhen the first count value reaches a first integer number, the firstinteger number indicative of the number of clock periods of theswitch-on duration; and during the switch-off duration, vary a secondcount value in response to the timer clock signal and generate a secondtrigger when the second count value reaches a second integer number, thesecond integer number indicative of the number of clock periods of theswitch-off duration.
 20. The method of claim 19, further comprising:during the switch-on duration, increasing the selection signal by athird integer number, the third integer number indicative of the numberof fractions of the clock period of the switch-on duration; during theswitch-off duration, increasing the selection signal by a fourth integernumber, the fourth integer number indicative of the number of fractionsof the clock period of the switch-off duration; setting the PWM signalto low, in response to the first trigger; and setting the PWM signal tohigh, in response to the second trigger.